1. Field of the Invention
This invention relates to optical recording materials and more particularly, to clathrate inclusion compounds which are adapted for use as a recording layer of optical recording media with which information is recorded or reproduced by irradiation of laser beams. The invention also relates to such optical recording media.
2. Description of the Prior Art
As is known in the art, optical recording media of the type in which information is recorded and reproduced by irradiation of laser beams have a great memory capacity and suffer little mechanical degradation because of recording and reproducing operations in a contact-free condition. Thus, such media have been accepted as those media which are adapted for storage and retrieval of a great quantity of information.
In general, the optical recording medium includes a substrate and a recording layer comprised of an optical recording material and formed on the substrate. On recording, a laser beam is converged on the recording layer to thermally change the material on the converged portions, thereby forming bits. For the reproduction of the recorded information, the difference in reflectance between the bit portion and the bit-free portion is detected and utilized.
In order for the successful recording and reproduction by the use of the optical recording medium of the type stated above, the optical recording material used as the recording layer should meet the requirements such as high sensitivity to heat, the capability of forming bits with a good configuration, a high reflectance in a laser beam wavelength, and good ability of storage.
Recently, attention has been directed to cyanine dyes as an optical recording material which can satisfy the above requirements. The cyanine dye is a so-called "perforation-type" optical recording material wherein the layer is perforated with heat generated on absorption of a laser beam. In fact, when used as an optical recording material, the cyanine dye has a number of advantages such as a great absorption in a laser beam wavelength with high sensitivity, small thermal conductivity and thermal diffusivity with the possibility that pits with a good configuration can be formed, a great reflectivity in a laser beam wavelength, high solubility in organic solvent and the like.
However, the cyanine dyes are disadvantageous in that the light fastness is poor. More particularly, when cyanine dyes are returned to a ground state after conversion to a triplet excitation state by application of light, oxygen is activated to produce oxygen of a singlet state. The singlet state oxygen is so reactive that it is reacted with the methine chain of the cyanine dye to change the structure. Accordingly, the cyanine dye is degraded with respect to the absorbance, reflectance and the like characteristics, making it difficult to perform good recording and reproducing operations.
In order to prevent the influence of the singlet state oxygen, it has been proposed to add substances capable of efficiently quenching the singlet state oxygen to the cyanine dye or to interrupt light with a short wavelength addition of UV absorbers or by coloration of a substrate.